As shown schematically in FIG. 1, a MEMS electrostatic actuator 10 (also known as a capacitive actuator) generally comprises two parallel electrodes, one electrode (the “bottom plate”) 12 of which is typically fixed to a substrate 14 and the other electrode (the “top plate”) 16 of which is suspended with a spring suspension 18 spaced by an air gap 20 above the bottom plate. A thin solid dielectric film 22 is deposited on the surface of the bottom plate 12 to prevent shorting. A voltage is applied between the two electrodes, creating an electric field that produces an attractive force between the two electrodes 12, 16. This force causes displacement of the top plate 16, until the top plate 16 makes contact with the dielectric 22 on the bottom plate 12. Once the voltage is removed, the top plate 16 returns to its original position under the restorative force of spring suspension 18.
A parallel plate actuator with a torsional architecture is described in P. Farnelli, et al., “A Wide Tuning Range MEMS Varactor Based on a Toggle Push-Pull Mechanism,” Microwave Integrated Circuit Conference, 474-477 (2008), and in F. Solazzi, et al., “Active Recovering Mechanism for High Performance RF MEMS Redundancy Switches,” Proceedings of the 40th European Microwave Conference, 93-96 (September 2010), both of which are hereby incorporated herein by reference.
One problem that arises in connection with MEMS capacitive actuators is stiction—static friction associated with adhesion of contacting surfaces. Stiction occurs when Van der Waals forces and the like cause the top plate to stick down on the dielectric on the bottom plate, even after the actuation voltage is removed.
A second problem arises when the top plate is landed on the bottom plate. At this point, the electric field across the thin solid dielectric that separates the two plates is very high. This causes electrostatic charge to be injected into the dielectric, a phenomenon known as dielectric charging. The charged dielectric has its own contribution to the electric field between the two plates, and it can cause undesirable effects: It can change the release voltage, the voltage at which the top plate returns to its original position. It can change the land voltage, the voltage at which the top plate lands on the bottom plate. It can shift the voltage that produces minimum capacitance away from OV. These issues are problems for repeatability and reliability, and they can cause early device failure.
A third problem that arises when the device is used as a variable capacitor for RF applications is self-actuation. When RF energy is applied to the plates, the RMS voltage of the RF signal can cause the top plate to move and potentially even land on the bottom plate, even if there is no DC actuation voltage present. This is especially a problem in high power RF applications, as it limits the power-handling specification for the device.
A fourth problem, similar to self-actuation, is self-latching. In RF applications, when the top plate is landed on the bottom plate, the RMS voltage on the RF signal on the bottom plate can hold the top plate down and prevent it from releasing, even when the DC actuation voltage is removed. This effectively prevents hot-switching, and presents a significant problem if the device is used in CDMA systems. The self-latching voltage is usually much lower than the self-actuation voltage.
A fifth problem is related to the flatness of the suspended top plate. Since the device is a parallel plate capacitor, both plates should ideally be flat and parallel. However, residual stresses from manufacturing can cause the top plate to curl. When the top plate is landed on the bottom plate, this curling creates an air gap between the two plates, greatly reducing on-state capacitance. This is illustrated in FIG. 2 which shows top plate 16 landed on solid dielectric layer 22 above bottom plate 12 on substrate 14. Top plate 16 is curled downward making contact peripherally with dielectric 22 over bottom plate 12 and leaving an air gap 20′ centrally between the regions of contact.
Other problems are slow switching speed and slow actuator settling time upon release. When the DC actuation voltage is removed, the top plate moves upward due to the restoring force of its spring suspension. Generally, this mass-spring system is underdamped, causing oscillation. The problem arises when the settling time for the oscillation is longer than the maximum switching time of the system, for example in transmit/receive switching application for GSM mobile handsets.
Another problem is one of system integration. In a typical RF handset application, there is no high voltage power supply available. Adding an off-chip supply is highly undesirable due to cost and board real estate. Therefore, the DC actuation voltage must be supplied by on-chip circuitry. It is a manufacturing challenge to co-locate MEMS and CMOS components. RF CMOS processes may provide good RF performance, but may be cost prohibitive. Also, they may not have the right component set required to generate an on-chip electrostatic supply. On the other hand, standard mixed signal processes provide poor RF performance. Moreover, there is a limit to the magnitude of the voltage that can be generated on the chip. As such, the actuation voltage of the MEMS structure must be below that limit.